1. Field of the Invention
This invention relates to a hybrid turbocharger for an internal combustion engine. More particularly, this invention relates to a high speed switch reluctance motor for use in a hybrid turbocharger.
2. Brief Description of the Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's power output without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of power as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of a reduced emissions.
Conventional turbochargers include a turbine housing connected to the engine's exhaust manifold, a compressor housing connected to the engine's intake manifold, and a center bearing housing coupling the turbine and compressor housings together. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft rotatably supported in the center bearing housing connects the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. As the compressor impeller rotates, it increases the air mass flow rate, airflow density and air pressure delivered to the engine's cylinders via the engine's intake manifold.
Turbochargers in which the turbine wheel is connected by a shaft to the compressor wheel tend to have a performance lag until the necessary exhaust energy is present to overcome the rotational inertia of the rotor assembly, and then reach the high rotation speeds required to provide the desired volume of air to the intake manifold. This performance delay is known as turbo lag. One solution to the problem of turbo lag is to couple the compressor wheel to an electric motor to drive the compressor stage until the necessary exhaust energy is present to power the turbine/compressor.
Switched reluctance motors may be used to drive the compressor. The switched reluctance motor (SRM) is a type of motor in which electric power is delivered to windings in the stator rather than the rotor. The electric current creates a magnetic field which pulls the rotor towards alignment with the stator poles. By switching the current from one set of poles of the stator to the next, the field is always ahead of the rotor and the rotor is pulled forward and thus rotates. The electric current is delivered to windings which surround poles in the stator portion of the motor. The rotor portion of the motor has no windings but instead has a core of soft magnetic material, typically laminated soft steel projections, generally in the shape of poles.